Visual feedforward control in human locomotion during avoidance of obstacles that change size.

The purpose of the current study was to understand how visual information about an ongoing change in obstacle size is used during obstacle avoidance for both lead and trail limbs. Participants were required to walk in a dark room and to step over an obstacle edged with a special tape visible in the dark. The obstacle's dimensions were manipulated one step before obstacle clearance by increasing or decreasing its size. Two increasing and two decreasing obstacle conditions were combined with seven control static conditions. Results showed that information about the obstacle's size was acquired and used to modulate trail limb trajectory, but had no effect on lead limb trajectory. The adaptive step was influenced by the time available to acquire and process visual information. In conclusion, visual information about obstacle size acquired during lead limb crossing was used in a feedforward manner to modulate trail limb trajectory.

Control of dynamic stability during adaptation to gait termination on a slippery surface.

An unexpected slip during gait termination results in a generalised slip response designed to regain stability and prevent a fall. With knowledge of and experience with a slippery surface, locomotor behaviour adapts to proactively diminish the effect of the slip and improve the reactive control during the slip. Our purpose was to examine the organisation of the adaptation to a slippery surface during gait termination. After receiving an unexpected slip during gait termination, participants (N = 8) experienced cued gait termination trials in which they were given knowledge of the surface characteristics (i.e., slippery or non-slippery). The observed strategy used to repeatedly stop on a slippery surface involves proactively diminishing the size of the slip perturbation through a flattened foot at heel contact, anterior shift of the COM, shorter step, stance leg extension and swing limb slowing, as well as improving the reaction to the slippery surface through decreased muscle activity and an appropriate decrease in the braking force generation. The implications of this research are that a combination of knowledge of and experience with a slippery surface enables proactive and reactive adjustments in behaviour to effectively and more safely stop walking on a slippery surface.

Gaze behavior during locomotion through apertures: the effect of locomotion forms.

The present study investigated spatio-temporal patterns of gaze fixations for passing safely through apertures. We focused on whether fixation patterns changed in response to changes in locomotion forms. Eight participants approached and passed through a narrow doorway using the following locomotion forms: normal walking, walking while holding a 63-cm horizontal bar with or without shoulder rotations permitted, and wheelchair use (63 cm wide). All participants were naïve to wheelchair use. The results showed that the fixation patterns were dependent on whether the locomotion form was walking or wheelchair use. In the three walking conditions, fixations were almost evenly directed toward the aperture and door edges at first; however, in the final phase, fixations were exclusively directed toward the center of the aperture. In contrast, in the wheelchair condition, fixations were directed more frequently toward the door edges throughout locomotion. These findings demonstrate that spatial-temporal patterns of fixation remain unchanged during walking through apertures, irrespective of the constraints on movement. The observed fixation patterns indicate that individuals appear to rely on optic flow to guide locomotion. However, the patterns of fixation are altered when they involve a completely novel task of locomotion, such as when using a wheelchair for the first time.

Behaviour and gaze analyses during a goal-directed locomotor task.

The objectives of the current study were: (a) to determine whether perception-action coupling controlled behaviours when walking through moving doors and (b) to determine how vision contributed to this behaviour. Participants (N = 6) walked along a 7-m path toward two motor-driven doors, which moved at rates ranging between 20 and 40 cm/s. Each door was independently driven such that both moved at the same velocity (symmetrical) or at different velocities (asymmetrical). The results showed that in both door movement conditions the participants controlled their approach velocity by slowing down prior to crossing the doors. The decrease in walking velocity produced greater velocity variability in the final stages prior to crossing the doors and high success rates. The results from the gaze behaviours showed that fixation durations were significantly longer when the doors moved asymmetrically, suggesting that the visual information from this unpredictable environment took longer to process. However, the fixation patterns were similar between the two door movement conditions. Regardless of the door movement condition, the participants spent about 60% of each trial fixating environmental objects (i.e., left door, right door, or aperture). The majority of fixations were directed towards one of the doors at the beginning of the trial and then shifted towards the aperture in the final phase. The participants were using perception-action coupling to control their behaviours in the final phase in order to steer locomotion through the aperture.

Control of dynamic stability during gait termination on a slippery surface in Parkinson's disease.

This study investigated how Parkinson's disease (PD) affects the ability to switch from locomotion to gait termination (GT) during planned and cued GT and examined the effect of PD on the integration of a reactive, balance maintenance strategy into voluntary GT. After a series of stops on a stable surface, eight participants with and 10 without PD stopped on a surface, which slid quickly and unexpectedly forward mimicking a slippery surface. PD caused instability during the completely voluntary nonslippery stops (P = 0.012) but not during the slippery stops, which required a reactive movement. The PD group walked slower [0.9-1.0 m/s vs. 1.3 m/s, respectively (P < 0.001)] with shorter steps during the first step of nonslippery GT (P = 0.016) and with wider steps during all steps of nonslippery GT (P

Locomotor avoidance behaviours during a visually guided task involving an approaching object.

Collision avoidance behaviours in situations where a collision may occur and one's planned movement is restricted, reveals that one's response is not as simple as a visual input producing some motor output. In this study, the participants (N=6) walked along a 9.5m path towards an air-filled human doll (180 degrees from their travel path) that would approach them on some trials. A spatial constraint (i.e. doorframe) was placed along the path and the participants had to determine if they could pass through the constraint prior to avoiding a collision or not. The constraint was set-up so that it was either at the theoretical collision point or 1.5m before or after the theoretical collision point. This study aimed to determine: (1) how the presence of a spatial constraint affects one's ability to perceive when to avoid a collision with an approaching object; (2) if the individuals use action parameters (i.e. velocity modifications, change in heading, etc.) in a consistent manner independent of the spatial constraint location and object's approach velocity; (3) if a consistent safety zone exists independent of the object's approach velocity. The results showed that the placement of the spatial constraint, but not the velocity of the object had a significant effect on the initiation of a change in heading. Participants used two-stage avoidance behaviour; change heading and then adjust walking velocity. The initial avoidance behaviour was initiated when the object was at a constant distance away (i.e. 3.73 m). Overall, it appears as though collision avoidance with approaching objects has cognitive as well as perceptual influences.

Visual information from the lower visual field is important for walking across multi-surface terrain.

Visual information concerning characteristics of the environment is critical for safe navigation. The purpose of this study was to determine the importance of vision from the lower visual field for negotiating multi-surface terrain. Ten healthy young adults and ten healthy older adults walked across a walkway where the middle portion consisted of solid, rock, slippery, compliant, tilt, and irregular surfaces (i.e. multi-surface terrain). Participants performed the walking trials with and without special glasses that blocked the lower visual field. Head pitch angle along with step parameters were measured. Young and older adults demonstrated increased mean and maximum head pitch angle downward when the lower visual field was blocked suggesting the importance of vision from this area when stepping on multi-surface terrain. In addition, young and older adults altered their gait pattern by reducing gait speed and step length when the lower visual field was blocked. These results suggest that information from the lower visual field is normally used when walking across multi-surface terrain. The results have implications for those individuals who wear multi-focal glasses and who use them while walking in complex environments, which may challenge balance.

Task-specific modulations of locomotor action parameters based on on-line visual information during collision avoidance with moving objects.

The objectives of this study were: (a) to determine if the control mechanism for interacting with a dynamic real environment is the same as in the virtual reality (VR) studies, and (b) to identify the action control parameters that are modulated to successfully pass through oscillating doors. The participants walked along a 14-m path towards oscillating doors (rate of change in aperture size = 44 cm/s and maximum aperture varied 70, 80, or 100 cm). The participants had to use vision to extrapolate what the aperture of the doors would be at the time of crossing and determine if a change in action parameters was necessary. If their current state did not match the required state then the participants made modifications to their actions. The results showed that individuals in a real environment used similar action modifications (i.e., velocity adjustments) as those seen in VR studies to increase success. Aside from the gradual velocity adjustments observed, there was an immergence of a different locomotor action parameter on some trials that was not seen in VR studies (i.e., shoulder rotations). These shoulder rotations occurred when the participants perceived that a velocity adjustment alone would not lead to a successful trial. These results show that participants use perception to control movement in a feedback rather than feedforward manner.

Age-related changes in gait for multi-surface terrain.

The main purpose of this study was to determine the effects of aging on the variability of gait over multi-surface terrain. An additional aim was to compare this variability to walking on solid ground. Ten young adults and 10 older adults walked along a walkway which contained a middle portion with multi-surface terrain. This multi-surface terrain consisted of solid, compliant, rocky, irregular, slippery, and uneven surfaces. We examined the effects of the multi-surface terrain and age on measures of variability including step length, step width, trunk pitch and roll, trunk centre of mass (tCOM) acceleration, and head acceleration. Step, trunk, and head variability were increased on the multi-surface terrain as compared to solid ground for both young and older adults (p<0.05). Older adults demonstrated larger medial-lateral tCOM acceleration RMS (p=0.0004) and trunk roll RMS (p=0.0001) when walking on the multi-surface terrain. In addition, older adults also walked more slowly (p=0.002) and took shorter steps (p=0.003). However, there were no age-related differences in step variability. The results suggest that multi-surface terrain poses a greater challenge to balance reflected by the increased variability, particularly in older adults.

Strategies used to walk through a moving aperture.

The objectives of the study were to determine what strategy (pursuit or interception) individuals used to pass through an oscillating target and to determine if individuals walked towards where they were looking. Kinematic and gaze behaviour data was collected from seven healthy female participants as they started at one of five different starting positions and walked 7 m towards an oscillating target. The target was a two-dimensional 70 cm aperture made by two-76 cm wide doors and oscillated between two end posts that were 300 cm apart. In order to quantify the objectives, target-heading angles [Fajen BR, Warren WH. Behavioral dynamics of steering, obstacle avoidance, and route selection. J Exp Psychol Hum Percept Perform 2003;29(2):343-62; Fajen BR, Warren WH. Visual guidance of intercepting a moving target on foot. Perception 2004;33:689-715] were calculated. Results showed that the participants used neither an interception nor a pursuit strategy to successfully pass through the moving aperture. The participants steered towards the middle of the pathway prior to passing through the middle of the aperture. A cross correlation between the horizontal gaze locations and the medial/lateral (M/L) location of the participants' center of mass (COM) was performed. The results from the cross correlation show that during the final 2s prior to crossing the aperture, the participants walked where they were looking. The findings from this study suggest that individuals simplify a task by decreasing the perceptual load until the final stages. In this way the final stages of this task were visually driven.

Balance control during continuous rotational perturbations of the support surface.

This research investigated the effects of continuous rotational perturbations of the support surface on postural control strategies adopted to maintain upright stance. Four different sinusoidal rotations of the support surface were employed: 0.5 Hz, at 2 degrees; 1.0 Hz at 1 degrees; 1.5 Hz at 4 degrees; and 2 Hz at 3 degrees. Thereby two different velocities of perturbation were obtained: 3.1 degrees s(-1) for 0.5 and 1 Hz, and 18.9 degrees s(-1) for 1.5 and 2 Hz. Results indicate that for the frequencies tested, the effect of the perturbation was attenuated. The amplitude of the body's center of mass (COM) displacement was reduced by adopting a multi-segmental strategy which employed anti-phase ankle and hip joint motion. Our results suggest that at least a two-link model of human stance is required to explain responses when the support surface is rotating.

The control of upright stance in young, elderly and persons with Parkinson's disease.

The aims of the present study are twofold: (1) to compare the postural control mechanisms of young and elderly people as well as in Parkinson's disease (PD) patients during quiet standing and (2) to assess the impact of a stooped posture on these mechanisms. All subjects were required to maintain both a side-by-side and a 45 degrees foot position. Elderly subjects performed a third condition where they were requested to mimic the stooped posture as adopted by PD subjects. The net centre of pressure (COP(net)) and centre of mass (COM) profiles in the anterior/posterior (A/P) and medial/lateral (M/L) planes were analyzed. The COP(net) signal was recorded from two force plates and was categorized in two mechanisms: an ankle mechanism (COP(c)) and a load/unload hip mechanism (COP(v)). The results showed similar postural control mechanisms in young, elderly and PD subjects. When the feet were side-by-side, the COP(net) was controlled by the ankle plantar/dorsiflexors (COP(c)) in the A/P direction, while by the hip abductor/adductors (COP(v)) controlled in the M/L direction. When the feet were in the 45 degrees position, both the ankle and hip mechanisms contributed to the COP(net). However, the PD subjects showed significant smaller RMS amplitudes compared to the elderly people in the 45 degrees foot position and in the stooped posture. These findings suggest that PD subjects resort to a stiffening strategy to control their balance in postural tasks that imply a mixed control (ankle and hip mechanisms) but have adapted to their stooped posture.

Balance during obstacle crossing following stroke.

Difficulty negotiating obstacles may contribute to the high falls rate following stroke. This study examined the impact of stroke on balance during obstacle crossing. Centre of mass (COM) and centre of pressure (COP) were measured as 12 stroke subjects and 12 unimpaired subjects stepped over a 4 cm high obstacle at self-selected speed. Unimpaired subjects also walked at speeds matched to their yoked stroke subject. Compared with unimpaired subjects at matched speed, at unaffected lead toe clearance, anterior-posterior (AP) separation between COM and COP increased in stroke subjects, which might indicate instability. Step lengths before and after the obstacle tended to be reduced which could increase the risk of losing balance forwards. The COM AP velocity was reduced at affected lead toe off following stroke, which may minimise instability. Following stroke the COM and COP were positioned more posteriorly during affected lead toe clearance, which might also assist stability.

Validating determinants for an alternate foot placement selection algorithm during human locomotion in cluttered terrain.

The goal of this study was to validate dynamic stability and forward progression determinants for the alternate foot placement selection algorithm. Participants were asked to walk on level ground and avoid stepping, when present, on a virtual white planar obstacle. They had a one-step duration to select an alternate foot placement, with the task performed under two conditions: free (participants chose the alternate foot placement that was appropriate) and forced (a green arrow projected over the white planar obstacle cued the alternate foot placement). To validate the dynamic stability determinant, the distance between the extrapolated center of mass (COM) position, which incorporates the dynamics of the body, and the limits of the base of support was calculated in both anteroposterior (AP) and mediolateral (ML) directions in the double support phase. To address the second determinant, COM deviation from straight ahead was measured between adaptive and subsequent steps. The results of this study showed that long and lateral choices were dominant in the free condition, and these adjustments did not compromise stability in both adaptive and subsequent steps compared with the short and medial adjustments, which were infrequent and adversely affected stability. Therefore stability is critical when selecting an alternate foot placement in a cluttered terrain. In addition, changes in the plane of progression resulted in small deviations of COM from the endpoint goal. Forward progression of COM was maintained even for foot placement changes in the frontal plane, validating this determinant as part of the selection algorithm.

Dopaminergic modulation of timing control and variability in the gait of Parkinson's disease.

The basal ganglia have been implicated in timing control, yet the nature of timing disturbances in Parkinson's disease (PD) is poorly understood. We evaluated the influence of timing cues on spatiotemporal aspects of gait control and its variability, and the impact of dopaminergic treatment on timing. Three separate groups: 19 PD (OFF state); 24 PD (ON state); and 30 control participants were tested. Participants walked on a computerized carpet at four randomized and metronome-controlled rates: self-paced, 60, 80, or 100 steps/min. To our knowledge, this is the first study to demonstrate that medicated PD patients had poorer timing control than patients withdrawn from medication and healthy participants when modulating timing to an external stimulus. Increased step-to-step timing variability and deficits in mean temporal gait characteristics revealed that the medicated PD group (in contrast to nonmedicated PD group) performed least like healthy participants. This was observable in externally-cued conditions, but not during self-paced gait. Similar to previous research, step length contributed to overall slowness in PD, while temporal characteristics of gait did not. Interestingly, healthy participants increased stride length with each increase in cue rate, whereas both PD groups locked their step length regardless of temporal demand. Step-to-step variability differences between PD and healthy (e.g. step and double-support time measurements) may be indicative of specific basal ganglia involvement in temporal control of gait.

Visual-vestibular interaction during goal directed locomotion: effects of aging and blurring vision.

Normal vision overrides perturbed vestibular information for the optimization of performance during goal directed locomotion, suggesting down-regulation of vestibular gain. However, it is not known if the responses to vestibular perturbation are accentuated when vision is impaired. Furthermore, both visual and vestibular systems deteriorate with age. It is not clear, however, how age-related decline in these sensory systems influences visual-vestibular interaction. Therefore, the dual purpose of the present study was to investigate the effects of aging and blurring vision, that simulated the consequences of cataracts, on visual-vestibular interaction. Young and healthy elderly walked to a target located straight ahead with either normal or blurring vision. On randomly selected trials vestibular system perturbation was achieved by applying transmastoidal galvanic vestibular stimulation (GVS). Two different galvanic stimulation intensities were used to provide insight into scaling effect of vestibular perturbation on locomotor performance and how age and vision influences this scaling effect. Maximum path deviation, frontal trunk tilt and postural coordination in the mediolateral direction were evaluated. The magnitude of the path deviation and the trunk tilt response were scaled to the magnitude of the vestibular perturbation in older adults independent of the visual condition. Older participants demonstrated increased coupling of the head and trunk segments irrespective of visual and vestibular perturbations. The results suggest that when visual information was available, the vestibular input reweighting was less effective in older individuals, as shown by the scaled responses to the GVS intensities and the inability to converge efficiently towards the target.

Keep looking ahead? Re-direction of visual fixation does not always occur during an unpredictable obstacle avoidance task.

Visual information about the environment, especially fixation of key objects such as obstacles, is critical for safe locomotion. However, in unpredictable situations where an obstacle suddenly appears it is not known whether central vision of the obstacle and/or landing area is required or if peripheral vision is sufficient. We examined whether there is a re-direction of visual fixation from an object fixated ahead to a suddenly appearing obstacle during treadmill walking. Furthermore, we investigated the temporal relationship between the onset of muscle activity to avoid the obstacle and saccadic eye and head movements to shift fixation. Eight females (mean +/- SD; age = 24.8 +/- 2.3 years) participated in this experiment. There were two visual conditions: a central vision condition where participants fixated on two obstacles attached to a bridge on the treadmill and a peripheral vision condition where participants fixated an object two steps ahead. There were two obstacle release conditions: only an obstacle in front of the left foot was released or an obstacle in front of either foot could be released. Only trials when the obstacle was released in front of the left foot were analyzed such that the difference in the two obstacle conditions was whether there was a choice of which foot to step over the obstacle. Obstacles were released randomly in one of three phases during the step cycle corresponding to available response times between 219 and 462 ms. We monitored eye and head movements along with muscle activity and spatial foot parameters. Performance on the task was not different between vision conditions. The results indicated that saccades are rarely made (< 18% of trials) and, when present, are initiated approximately 350 ms after muscle activity for limb elevation, often accompanied by a downward head movement, and always directed to the landing area. Therefore, peripheral vision of a suddenly appearing obstacle in the travel path is sufficient for successful obstacle avoidance during locomotion: visual fixation is generally not re-directed to either the obstacle or landing area.

Travel path conditions dictate the manner in which individuals avoid collisions.

The ability to perceive the motion of approaching objects and to make appropriate adjustments to avoid collisions is an essential component of safe locomotion through the environment. No previous study has looked at actual behavioural responses in a realistic collision avoidance task. We investigated individuals' collision avoidance behaviour with an approaching object in order to understand the visual information used to accomplish the task. We found that when individuals were walking towards a goal without any restrictions, a change in travel path occurred at the same time, independent of the object's approach velocity. However, the lateral rate of change in the travel path was significantly slower for the slower approach velocity. We found that when a restriction was present along the travel path, individuals were exposed to the object's properties for longer and a change in travel path occurred later for slowest approaching velocities than for the fastest approaching velocities. The results suggest that individuals are capable of determining TTC from an approaching object during actual self-motion, but the motor parameter it modulates is different for different circumstances.

Locomotion through apertures when wider space for locomotion is necessary: adaptation to artificially altered bodily states.

The objective of this study is to describe the adaptability of the central nervous system to safely cross a narrow aperture when the space required for passage is transiently extended with external objects under different locomotor constraints. In one of four locomotion forms (normal walking, walking while holding a 63-cm horizontal bar with or without rotating the shoulders to cross a door opening, and wheelchair use), nine participants were asked to pass through an aperture created by two doors (the relative aperture widths were 1.02, 1.10, and 1.20 times their maximum horizontal dimension under each form of locomotion) without a collision. The kinematic analyses showed that, when the participants rotated their shoulders while walking and holding a bar, virtually the same locomotor patterns as those during normal walking were observed: shoulder rotation was regulated well in response to the width of an aperture, and no collisions occurred. When shoulder rotations were restricted while walking and holding a bar or using a wheelchair, a large reduction in the speed of movement was observed as the participants approached the door, and, furthermore, the modulation in speed was dependent on the width of the aperture. In addition, the participants crossed at the center of aperture more accurately; nevertheless, collision sometimes occurred (more frequently, during wheelchair use). These findings reveal that movement constraints on shoulder rotation are likely to be a critical factor in determining whether quick and successful adaptation takes place.